1. Field of the Invention
This invention relates to electronic article surveillance systems and more particularly it concerns arrangements for detecting unauthorized removal of surveillance system security tags from articles of protected merchandise.
2. Description of the Prior Art
Electronic article surveillance systems for protecting articles of merchandise from theft are well known. In all of these systems the articles of merchandise to be protected have a security tag containing a "target" element attached to them and when the article is carried from a protected area, such as a store, an antenna at the exit from the protected area senses the target element and sounds an alarm. When the article of merchandise is purchased, the store clerk either deactivates the target or removes the security tag so that the merchandise can be taken from the protected area without activating the alarm.
Systems have been proposed which utilize so called "active" targets which contain their own power and give off distinctive electromagnetic signals or disturbances which are detected when they are carried past sensing antennas at the store exit. U.S. Pat. No. 2,474,271 describes one such system.
Most systems however, use "passive" targets or responders which respond to electromagnetic radiation supplied from an interrogation antenna at the store exit and produce distinctive electromagnetic disturbances which are then detected. U.S. Pat. Nos. 3,493,955, 2,774,060, 3,500,373, 4,321,586 and 4,623,877 and 4,642,613 describe such responder systems.
It is important that the target not be deactivatible or removable from the merchandise except by special means controlled by the store clerk. Various devactivation and removal arrangements have been proposed and utilized. U.S. Pat. No. 3,810,147 proposes to provide a fusible link in a resonant circuit target which, when subjected to radiation at the appropriate power and frequency, would melt the link and change the distinctive resonant characteristics of the target so that it cannot be detected.
U.S. Pat. Nos. 3,747,086, 3,820,103 and 3,820,104 describe the provision of high coercivity magnetic elements mounted on a soft magnetic strip target so that, when the elements are magnetized they prevent the soft magnetic strip from producing characteristic harmonics of an interrogation field.
U.S. Pat. Nos. 3,858,280, 3,911,534, 4,531,264 and 4,590,461 describe specially detachable locking arrangements for security tags which allow them to be removed from the merchandise only with a special tool controlled by the store clerk. The electromagnetic characteristics of the targets contained in these tags is not altered and they may be reused, after removal, to protect other articles of merchandise. These security tags are generally in the form of plastic wafers and are fastened to the merchandise by means of a tack-like fastening element whose shank passes through the merchandise and enters into a locking mechanism on the tag. The locking mechanism can be released by a special tool, e.g. a special magnet, controlled by the store clerk.
A problem has arisen in connection with removable security tags in electronic article surveillance systems. Although the locking mechanism can be released legitimately only with a special tool, it may be possible, by continued working with a large and powerful tool, for example, pruning shears or a heavy duty cable cutter, eventually to destroy the locking mechanism and then remove the security tag. Although it would not be possible to use such tools in the public areas of a store without arousing suspicion, it is possible to use them without detection in the privacy of a dressing room or a rest room in the store. Thus a thief can bring merchandise into the store dressing room and, without detection, work with a large tool for as long as necessary to destroy the security tag locking mechanism and then remove the tag and target from the merchandise. The protection of merchandise in private areas such as in dressing rooms and rest rooms has been a difficult problem because a certain degree of privacy must be accorded to store patrons in those areas.